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Thread: Dual tap OT quandry

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    Supporting Member Randall's Avatar
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    Dual tap OT quandry

    Customer brings me a 2016 Blues Jr and wants every mod known to man done to it. One of which is to install a David Allen OT he supplied with 4 and 8 ohm taps. He wants to be able to run just the stock speaker, but wants an additional speaker jack installed so he can plug an extension speaker in if he wants. What would be the best way to achieve this? I'm thinking wiring two jacks in parallel and wiring a SPDT switch between them and the two OT taps?

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    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    I think this is the safest way to go because it's the most easily understood. Mesa does a thing with some of their amps where the jacks are labeled for the ohm load. It's stupid. Another system I've seen would be switching jacks where the main speaker is always in the designated jack and plugging in the extension jack halves the secondary ohms. But then what if you ONLY want the extension cab? Will the guy know or remember to unplug the main speaker and lug the extension cab into THAT jack? Too much to go wrong in the hands of most guitar players.

    I vote parallel with a switch. Then it's on the customer. Niether jack is "wrong" and the switch should be adjusted for the load. They KNOW they're changing the load when they fuss with the speakers, so...

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    Quote Originally Posted by Chuck H View Post
    I think this is the safest way to go because it's the most easily understood. Mesa does a thing with some of their amps where the jacks are labeled for the ohm load. It's stupid. Another system I've seen would be switching jacks where the main speaker is always in the designated jack and plugging in the extension jack halves the secondary ohms. But then what if you ONLY want the extension cab? Will the guy know or remember to unplug the main speaker and lug the extension cab into THAT jack? Too much to go wrong in the hands of most guitar players.

    I vote parallel with a switch. Then it's on the customer. Niether jack is "wrong" and the switch should be adjusted for the load. They KNOW they're changing the load when they fuss with the speakers, so...
    This is how Marshall does it. I personally like the speaker out to be the speaker out, and using a switch to set the desire impedance as well.
    But i prefer the "main/extension" switching jack set up because you can use the main jack as a shorting jack when there is no external load plugged in. While not ideal, a short across the output of a tube amp can save the output transformer and tube sockets if there is no load plugged it and the output goes open. This protects the player (and... sometimes,.. the technician) from screwing up and not connecting the speaker for whatever reason.
    Not my favorite method of protection, but if I had to choose between the two.
    Multiple designated output jacks like Mesa, and others do it makes no sense to me. With Mesa, it doesn't really matter though. Those jacks save mesa customers from having to deal with the 32 switches and 12 knobs that would be there if the speaker jacks weren't.

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    Supporting Member Randall's Avatar
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    My idea is to put the switch between the jacks so that it points to the jack. One side points the main jack, and the other side points to the jack when you plug in an extension cab.

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    Last edited by Randall; 11-24-2019 at 09:47 PM.
    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Eh. It easier to wire parallel jacks when you don't jam the impedance switch between them. For any difference it makes I don't think it's worth the trouble. JM2C. Whatever you decide I think SoulFetish is right to mark the main jack and the extension jack and make the main jack shorting.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    It helps to have the back panel silkscreened, labeling each accordingly. But who the hell has time (or equipment) for that. You could put a red washer on the main just so they know. But, the other great thing about wiring the main speaker for shorting, is that if they plug into the extension jack, they don't get any sound.
    If that happens, we can only hope the wind is at their back that day, and by process of elimination, the guitarist figures out that it's the other jack.

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    I prefer a safety resistor across the secondary, about 20-30 X the nominal load value.
    The low value of the feedback resistor network in most BF/SF amps might act as such a safety resistor. Shorting jacks are no use if the speaker blows, cable breaks etc, but combined with a safety resistor, they seem to provide a resilient ‘belt and braces’.
    With only 2 impedances, I prefer to use 3 jacks, rather than 2 parallel jacks and a switch; on the basis that the fewer things to go wrong in the secondary circuit the better (prime example being the dodgy removable impedance selector Marshall used to use). That excludes the shorting jack option but I’m doubtful that ‘ELxx’ tubes in fixed bias can cope with a shorted load any better than an open load.

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    Early Marshall JTM45s (RS OT, no imp. selector) had a 1k safety resistor.

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    Wow, that's interesting, I've not seen that reported before; do you think they left the factory like that?

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    do you think they left the factory like that?
    Yes, I think so. They look original. I have a collection of pictures from Plexi Palace and from http://marstran.com/Historic%20Data%20Base.htm (print-outs) and the 1k CC safety resistor can be seen on several amps. My '66 JTM50 with Drake OT and impedance selector doesn't have it. Older schematics don't show it either but I have never seen a schematic with the RS OT.

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    Last edited by Helmholtz; 11-26-2019 at 02:48 PM.
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  11. #11
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    This is a blues Jr, and I don't think (or see a reason) why the main speaker would be disconnected. This guy is savvy and just wants to be able to add a cab in certain rooms. I know the switch will work, but I have also seen reference on billmaudio,com that there is a way to wire up an aux speaker jack so that it switches to 4 ohms automatically. Anyone know how that works?

    The main speaker jack is wired for shorted when unplugged.

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    It's weird, because it WAS working fine.....

  12. #12
    Supporting Member Jazz P Bass's Avatar
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    I have attached the Fender HR series jack switching setup.
    Hot_Rod_DeVille_Spkr Jack.pdf

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  13. #13
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    Thanks Jazz, I suspected it was something along those lines. That's what I will do. Need to figure out the correct switching jack, preferably an open switchcraft type.

    This should do the trick. http://switchcraft.com/Product.aspx?ID=2701

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    Last edited by Randall; 11-26-2019 at 03:32 AM.
    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Alright. Assuming Randall has worked out how to proceed...

    What is the purpose of bypassing the 1 ohm resistor on the power tube cathodes in the schematic with a diode.?. Is that to limit the affect AC standing on top of the 1 ohm resistance may have on the mV reading? Since the measurement here is usually taken when the amp is at idle there shouldn't be any to speak of, right? Is it even significant enough for this extra diode?

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Supporting Member The Dude's Avatar
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    That's a conundrum for me also. I've seen diodes used for clamping or constant gm circuit/purposes (with a larger value cathode resistor), but the 1 ohm parallel resistor seems to make the diode a moot point. Maybe it's not intended for normal circuit operation, but rather if the resistor starts to open?

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    Quote Originally Posted by Chuck H View Post
    Alright. Assuming Randall has worked out how to proceed...

    What is the purpose of bypassing the 1 ohm resistor on the power tube cathodes in the schematic with a diode.?. Is that to limit the affect AC standing on top of the 1 ohm resistance may have on the mV reading? Since the measurement here is usually taken when the amp is at idle there shouldn't be any to speak of, right? Is it even significant enough for this extra diode?
    They do that to save the resistor from opening in case of a short. If the current through the resistor causes the voltage across it to rise, the diode will forward bias and clamp the voltage, and present a low impedance path in parallel. In theory, this should prevent the max power rating of the resistor from being exceeded

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    Quote Originally Posted by The Dude View Post
    That's a conundrum for me also. I've seen diodes used for clamping or constant gm circuit/purposes (with a larger value cathode resistor), but the 1 ohm parallel resistor seems to make the diode a moot point. Maybe it's not intended for normal circuit operation, but rather if the resistor starts to open?
    Right, it’s reverse biased in normal operation

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  18. #18
    Supporting Member The Dude's Avatar
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    The diode would always be forward biased. Unless......, for some very strange reason, your cathode voltage went negative. There just wouldn't be enough bias for it to do anything unless the resistor started to open.

    Edit: Re-thinking: The resistor is 1W 1 Ohm- so 1V 1A for a hot minute. I guess the diode would start conducting before the resistor reached max ratings...... So, the diode probably eventually shorts but the amp still works (shorted diode and all) after you change the bad tube and blown fuse?

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    Last edited by The Dude; 11-26-2019 at 06:44 AM.
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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by The Dude View Post
    The diode would always be forward biased. Unless......, for some very strange reason, your cathode voltage went negative. There just wouldn't be enough bias for it to do anything unless the resistor started to open.

    Edit: Re-thinking: The resistor is 1W 1 Ohm- so 1V 1A for a hot minute. I guess the diode would start conducting before the resistor reached max ratings...... So, the diode probably eventually shorts but the amp still works (shorted diode and all) after you change the bad tube?
    yes, you're right. My bad. it would just be biased off.

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    Quote Originally Posted by pdf64 View Post
    Wow, that's interesting, I've not seen that reported before; do you think they left the factory like that?
    Here is an example:

    Click image for larger version. 

Name:	JTM45gut.jpg 
Views:	24 
Size:	623.2 KB 
ID:	56064

    More here: http://marstran.com/Historic%20Data%20Base.htm

    e.g. http://marstran.com/64%201015.htm

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    Last edited by Helmholtz; 11-26-2019 at 02:51 PM.
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    Supporting Member loudthud's Avatar
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    The purpose of the diode is to limit the Voltage at the back panel test point (where the user can touch it) in the case that the resistor fails open for any reason.

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    Don't forget the joker g1's Avatar
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    That makes sense. I had thought they might be worried about exceeding heater to cathode ratings if the resistor opened, but safety regs. and liability would be a much bigger deal.

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  23. #23
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by loudthud View Post
    The purpose of the diode is to limit the Voltage at the back panel test point (where the user can touch it) in the case that the resistor fails open for any reason.
    That's another great reason for those resistors, perhaps a better one. But in the Fender HR amps there are no back panel test points for the user to come into contact.

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    Without the diode, in the case of the fuse value being too high and a power tube shorting (neither of which happen often, of course), the resistor might 'fuse' and save the OT / PT etc from damage. Similar to 1W screen grid resistors.

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  25. #25
    Don't forget the joker g1's Avatar
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    This is a little anecdote I've shared before, but it is possible the diode got implemented for this reason and just got continued on because 'that's how we do it now'.

    This first mass-produced amp with 'user' bias adjust was the Fender "The Twin" released in the late 80's. The rear panel board had the test point receptacles, the bias trim pots, and the 1R cathode resistors. They did NOT have the diodes originally. Bad tubes would fry the 1R's, and often burn the board, the receptacles, and sometimes even the pots along with them. Lots of warranty claims for Fender I'm sure (we had several). The fix was to install the diodes across the resistors. It was a service bulletin and the instruction was that they were to be installed on ALL units that were in for warranty repair. Somebody that has all the service bulletins would have it.
    The next revision of that model had the diodes 'stuffed' as they say. As far as I know all Fender product since that has those sense resistors has the diodes in parallel.

    Maybe at this time someone realized the safety hazard loudthud mentioned, but I thought I'd relate the story due to them being used in models that don't have user accessible adjust, which SoulFetish brought up.
    I do agree with pdf64 that having the diodes negates the fusing function, which seems counter-intuitive.

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    Bent Member Chuck H's Avatar
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    Glad I asked

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Here is the Service Bulletin.

    https://music-electronics-forum.com/...1&d=1575168856
    Attached Files Attached Files

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  28. #28
    Supporting Member Randall's Avatar
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    Looking at the schematic in post #12, the grey negative feedback wire is connected to the 4 ohm tap, it doesn't appear to be employed when using just the main speaker. Why is this the case? I am wanting to impliment this switching scheme in a Blues Jr.

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    Supporting Member The Dude's Avatar
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    The 4 ohm tap is no longer used for speaker output, but it can still be used for NFB.

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    "The 4 ohm tap is no longer used for speaker output, but it can still be used for NFB. "

    Ah, I see. So is the reverse OK as well, as in leaving NFB connected to the 8 ohm tap in the Blues Jr? There will still be NFB in 4 ohm mode?

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    You betcha.

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    Lifetime Member Enzo's Avatar
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    The signal is present on all the taps whether you use the tap or not. NFB is taking the output signal and feeding it back. The different taps have of course different levels of signal. They chose the one tap. You can connect the speaker load to any appropriate tap.

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    Supporting Member Randall's Avatar
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    "The signal is present on all the taps whether you use the tap or not."

    Got it! Thank you.

    I know this is a lot of bother to add an aux speaker jack, but plug and play with a two tap OT is what this customer wants. He's served two tours of Viet Nam, then worked in the NSA, then NYPD, then played guitar in Otis Rush's band, then became his road manager because they were getting ripped off, so he carried always. So he gets what he wants!

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by Randall View Post
    he carried always. So he gets what he wants!
    Quote Originally Posted by Randall View Post
    wants every mod known to man done to it.
    You're going to be busy for awhile.

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    Supporting Member Randall's Avatar
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    Let's see, PT, OT, tone stack, bias adjust, aux speaker switching jack, input jack, filter caps, preamp HV mod, nickb replacement tube board, standby switch, tubestore.com premium tube set. The man knows what he wants. And when it is finished and he hears it, a different speaker may be in play as he has eluded.

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